richardstartin commented on a change in pull request #8016:
URL: https://github.com/apache/pinot/pull/8016#discussion_r794377855



##########
File path: 
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/utils/nativefst/utils/RealTimeRegexpMatcher.java
##########
@@ -0,0 +1,156 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.segment.local.utils.nativefst.utils;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Queue;
+import java.util.Set;
+import org.apache.pinot.segment.local.utils.nativefst.automaton.Automaton;
+import 
org.apache.pinot.segment.local.utils.nativefst.automaton.CharacterRunAutomaton;
+import org.apache.pinot.segment.local.utils.nativefst.automaton.RegExp;
+import org.apache.pinot.segment.local.utils.nativefst.automaton.State;
+import org.apache.pinot.segment.local.utils.nativefst.automaton.Transition;
+import org.apache.pinot.segment.local.utils.nativefst.mutablefst.MutableArc;
+import org.apache.pinot.segment.local.utils.nativefst.mutablefst.MutableFST;
+import org.apache.pinot.segment.local.utils.nativefst.mutablefst.MutableState;
+import org.roaringbitmap.IntConsumer;
+
+public class RealTimeRegexpMatcher {
+  private final String _regexQuery;
+  private final MutableFST _fst;
+  private final Automaton _automaton;
+  private final IntConsumer _dest;
+
+  public RealTimeRegexpMatcher(String regexQuery, MutableFST fst, IntConsumer 
dest) {
+    _regexQuery = regexQuery;
+    _fst = fst;
+    _dest = dest;
+
+    _automaton = new RegExp(_regexQuery).toAutomaton();
+  }
+
+  public static void regexMatch(String regexQuery, MutableFST fst, IntConsumer 
dest) {
+    RealTimeRegexpMatcher matcher = new RealTimeRegexpMatcher(regexQuery, fst, 
dest);
+    matcher.regexMatchOnFST();
+  }
+
+  // Matches "input" string with _regexQuery Automaton.
+  public boolean match(String input) {
+    CharacterRunAutomaton characterRunAutomaton = new 
CharacterRunAutomaton(_automaton);
+    return characterRunAutomaton.run(input);
+  }
+
+  /**
+   * This function runs matching on automaton built from regexQuery and the 
FST.
+   * FST stores key (string) to a value (Long). Both are state machines and 
state transition is based on
+   * a input character.
+   *
+   * This algorithm starts with Queue containing (Automaton Start Node, FST 
Start Node).
+   * Each step an entry is popped from the queue:
+   *    1) if the automaton state is accept and the FST Node is final (i.e. 
end node) then the value stored for that FST
+   *       is added to the set of result.
+   *    2) Else next set of transitions on automaton are gathered and for each 
transition target node for that character
+   *       is figured out in FST Node, resulting pair of (automaton state, fst 
node) are added to the queue.
+   *    3) This process is bound to complete since we are making progression 
on the FST (which is a DAG) towards final
+   *       nodes.
+   */
+  public void regexMatchOnFST() {

Review comment:
       Sorry to resurrect this thread but none of the underlying issues 
(profuse blank lines) have been addressed in this class.




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